A sol-gel method using tetraethoxysilane and acetic anhydride:: Immobilization of cubic μ-oxo Si-Ti complex in a silica matrix

A sol-gel procedure using acetic anhydride as a substitute for water using a condensing reagent for the tetraethoxysilane (TEOS) to silica conversion is reported. The mixed solution obtained from the reaction of TEOS and acetic anhydride at 110 degreesC was allowed to stand in air for several weeks, resulting in gel formation of the solution. This prepared silica was mesoporous based on the nitrogen adsorption-desorption isotherms. Even when the gelation was performed in the presence of a large amount of benzene, silica was also obtained. The immobilization of a cubic mu-oxo Si-Ti complex, which is a good homogeneous catalyst for the epoxidation of olefins, in a silica matrix was achieved by using this technique. Although the complex readily afforded TiO2 anatase on treatment with water, no free TiO2 phase was found in the solid obtained by this procedure according to XRD, DR-UV-vis, and Raman spectra. The titanium was highly dispersed in the silica matrix. On the other hand, a solid prepared by the same procedure using titanium tetra-isopropoxide as the titanium source contained the TiO2 anatase phase. The obtained SiO2-TiO2 mixed oxide solid was an effective heterogeneous catalyst for the highly efficient epoxidation of cyclohexene in the liquid phase by tert-butylhydroperoxide.